Many methods have been used and measures taken for preventing listening to information which is sent in the form of electrical or optical signals between two points.
One method is to encode the information which is to be transmitted in the transmitter unit and then decode this information in the receiver unit. This results in being able to use a relatively cheap transmission medium, but the encoding equipment is generally expensive and decoding by unauthorized persons is possible.
One measure is to mechanically or electromechanically protect the cable against unauthorized tampering or listening. This can also result in large costs, e.g. for mechanical protection by means of lead pipes, and does not remove the risk of listening with the aid of mechanical tampering.
A further known method consists of integrating electrical conductors into the fiber cable transmitting the information. When the cable is tampered with the capacitance between the electrical conductors is altered. If an alteration is detected an alarm can be sent, resulting in that the information transmission can be stopped. Although this method results in sure protection against listening, high costs are unavoidable.